Cryogenic fluid transfer and storage

ABSTRACT

A method of transferring a cryogenic fluid comprises passing a cryogenic fluid through a flexible conduit having a wall formed of a first layer of a porous polymeric ( 12 ) material and a second layer formed of an impermeable material ( 14 ).

FIELD OF THE INVENTION

[0001] This invention relates to methods of transferring and storingcryogenic fluids, and in particular to the use of flexible conduits andcontainers for transfer and storage of such fluids.

BACKGROUND OF THE INVENTION

[0002] Vacuum and dry gas insulated tubes are typically used totransport or store cold liquids or liquids with a low heat ofvaporisation. The coaxial design of these transfer tubes reduces thewarming rate of the cold liquid and results in a reduced exteriortemperature. The transfer tubes usually consist of two straight,corrugated or convoluted stainless steel tubes mounted one over top ofthe other. The use of multiple tubes provides some degree of insulationto help maintain low temperature liquids in a liquid state. The use ofcorrugations or convolutions lends somewhat increased flexibility, thatis a reduced bending radius, to the construction. A protective stainlesssteel mesh is often applied to the outer surface of the transfer tube.Overall, these transfer tubes suffer from numerous problems, includingpoor bend radius, excessive weight and size, and prolonged time todeliver cold liquids due to the initial cooling of the tubing by theliquid which is necessary before the liquid may pass through the tubingwithout significant vaporisation.

[0003] Alternative tubes in the prior art are much like the tubesdescribed above except that they do not provide a coaxial insulatingspace. Consequently, they do not provide the same insulating benefits.These tubes are typically used to deliver cold liquids over relativelyshort distances, such as delivering liquids from a storage tank. Thesetransfer tubes also suffer from a poor bend radius, large mass,prolonged time to deliver cold liquids and excessive frost accumulationon the outer surface of the tube and subsequent pooling of water in thevicinity after thawing. The tubes may also become brittle in use, and ifused to carry cryogenic fluids under pressure there may be a risk that atube may rupture, the resulting fragments of material and pressurisedleaking fluid presenting a hazard to operators in the vicinity.

[0004] U.S. Pat. No. 4,745,760 to Porter (NCR Corporation) discloses acryogenic fluid transfer conduit. The conduit transfers the fluidthrough an impermeable tube from a cryogenic reservoir to an enclosurefor cooling an integrated circuit, and its coaxial channel is used toreturn the fluid to the reservoir. This apparatus relies on the fluiddelivered out of the end of the tube to be re-directed into the coaxialspace for improved insulative properties.

[0005] A closed ended surgical cryoprobe instrument is described in U.S.Pat. No. 5,520,682 to Baust et al. This patent teaches the use of aclosed system to chill the end portion of a surgical instrument. Animpermeable inner tube is provided to deliver cooling fluid, with nofluid delivered outside of the chambers of the device.

[0006] U.S. Pat. No. 4,924,679 to Brigham et al. describes an insulatedcryogenic hose. A fluid that liquefies or solidifies at cryogenictemperatures fills the coaxial space of the article of this invention toimprove insulation, but at the cost of loss of overall flexibility ofthe tube.

[0007] Various polymers are known to be useful under low temperatureconditions such as 77° Kelvin (the temperature at which Nitrogen willremain liquid at atmospheric pressure). For example, porouspolytetrafluoroethylene (PTFE) is known to retain strength andflexibility at low temperatures, particularly in the form of porousexpanded PTFE (ePTFE) constituted by nodes interconnected by fibrils asdescribed in U.S. Pat. No. 3,953,566 to Gore. Such ePTFE, however, isnot normally suitable for the transport or storage of cryogenic liquidsbecause of its porosity, which allows cryogenic liquids to have readypassage into and through the ePTFE material.

[0008] Temperature gradients affecting materials used in systems such asthose involving cryogens are such that thermal expansion and contractioneffects may cause early mechanical failure in components. Preferredembodiments of this invention relate to materials that retainflexibility and strength at low temperatures, particularly cryogenictemperatures, such as 77 Kelvin.

SUMMARY OF THE INVENTION

[0009] The various aspects of the invention take advantage of theadvantageous properties of porous polymeric materials, particularlyporous polytetrafluoroethylene (PTFE).

[0010] One embodiment of the present invention relates to a method oftransferring a cryogenic fluid, the method comprising passing acryogenic fluid through a flexible conduit having a wall formed of afirst layer of a porous polymeric material and a second layer formed ofan impermeable material.

[0011] It has been found that this method compares favourably withconventional methods of transferring cryogenic fluids. As describedbelow, the use of a porous polymeric material to form at least a portionof the wall of the conduit has numerous surprising benefits, includingrelatively low mass, increased flexibility, and improved insulation. Theuse of the preferred fluoropolymers also enables the design of moreflexible tubes that can also withstand more flexural stresses prior tofailure.

[0012] The impermeable material may be selected from a wide range offlexible materials having appropriate low temperature characteristics,including polymeric materials, such as ethylene-polypropylene copolymer(EPC), polyester-based materials, polyvinylchloride (PVC), andfluoropolymers such as PTFE, fluorinated ethylene propylene (FEP),perfluoroalkoxy polymer (PFA) and blends and composites thereof.

[0013] Preferably, the porous polymeric material is a porousfluoropolymer, and porous expanded PTFE (ePTFE) is a particularlypreferred material because of its flexibility at cryogenic temperatures.

[0014] Preferably, the first layer is selected to have a heat capacityof less than 2.251×10⁶ kJ/m³K. The relatively low heat capacity resultsin the first layer being cooled more rapidly to cryogenic temperatureson flow of fluid through the conduit being initiated. As a result, thereis less production of gaseous cryogenic fluid on the fluid firstencountering the relatively warm conduit, and flow of fluid through theconduit may commence more rapidly. The preferred expanded PTFE has arelatively low heat capacity, determined by its density, and is lessthan 2.251×10⁶ kJ/m³K, the heat capacity of unexpanded PTFE.

[0015] According to another aspect of the invention, there is provided amethod of transferring a cryogenic fluid between two relatively movablelocations, the method comprising passing a cryogenic fluid through aflexible conduit having a wall formed of a first layer of a porouspolymeric material and a second layer formed of an impermeable material.

[0016] The ability of the present invention to transfer cryogenic fluidthrough a flexible conduit, facilitates the transfer of cryogenic fluidbetween two relatively movable locations, such as supplying cryogenicfluid from a cryogenic fluid source to a vibrating machine or a machinehaving a moving tool head or movable robot arm.

[0017] According to a further aspect of the invention, there is provideda method of storing a cryogenic fluid, the method comprising placing acryogenic fluid in a container having a wall formed of a first layer ofa porous polymeric material and a second layer formed of an impermeablematerial.

[0018] As with the fluid transfer aspects of the invention describedabove, the invention offers numerous advantages in the storage ofcryogenic fluids, including the ability to store and transport cryogenicfluids in flexible containers.

[0019] The invention also relates to a method of insulating a cryogenicfluid container having a wall formed of a first layer of an impermeablematerial, the method comprising providing the wall with a second layerof a porous polymeric material.

[0020] While the impermeable layer provides for containment of thecryogenic fluid, the porous polymeric material may provide effectiveinsulation and structural strength, without detracting from desirablephysical and structural attributes, such as flexibility and low mass.

[0021] The second layer of porous polymeric material may be providedeither internally or externally of the first layer, and indeed in someembodiments may be provided both internally and externally.

[0022] Another aspect of the invention relates to a flexible cryogenicfluid transfer conduit comprising a wall formed of a first portion of aporous polymeric material and a second portion comprising a plurality oflayers of coiled impermeable sheet.

[0023] A further aspect of the present invention provides a flexiblecryogenic fluid transfer conduit comprising a wall formed of a innerfirst portion comprising a plurality of layers of porous polymeric sheetand an outer second portion comprising a plurality of layers ofimpermeable sheet, the impermeable sheet being of smaller thickness thanthe porous polymeric sheet.

[0024] Impermeable material tends to be relatively inflexible,particularly at cryogenic temperatures, and thus the layers ofimpermeable sheet are of relatively small thickness, to preserve as muchflexibility as possible. Also, the impermeable material may be spacedfrom direct contact with the cryogenic liquid by the inner first portionof porous material, and thus may not experience the same extreme lowtemperatures that the porous material experiences. The invention alsofacilitates such a construction, as many of the physical and structuralattributes of the conduit may be provided by the relatively flexibleporous material, the main function of the impermeable material simplybeing to contain the fluid.

[0025] A still further aspect of the present invention relates to aflexible cryogenic fluid transfer conduit comprising a wall formed of afirst portion of porous polymeric material and a second portion ofimpermeable material, the conduit having a diameter of less than 25.4mm.

[0026] In another aspect of the present invention there is provided aflexible cryogenic fluid transfer conduit comprising a wall formed of afirst portion of a seamless porous polymeric tube and a second portionof impermeable material.

[0027] The seamless porous polymeric tube, typically formed by extrudingmaterial in tube form, provides a convenient base tube for the conduit.

[0028] One aspect of the present invention relates to a flexiblecryogenic fluid transfer conduit comprising a wall formed of a firstportion of porous polymeric material and a second portion of impermeablematerial, at cryogenic temperatures the conduit having a flexibility, asdetermined by the bend diameter test set out below, of 20 to 1 or less.

[0029] Preferably, at cryogenic temperatures, the conduit has aflexibility of 10 to 1 or less, that is the bend diameter of the conduit(the diameter of the cylinder about which the conduit is wrapped) may beless than 10 times the diameter of the conduit. Most preferably, theconduit has a flexibility of 5 to 1 or less.

[0030] The provision of a conduit with a wall having such a flexibility,made possible in part by the presence of a wall portion of porousmaterial, increases the ease and convenience of use of the conduit.

[0031] Aspects of the invention relate to a flexible cryogenic fluidtransfer conduit comprising a wall formed of a first portion of porouspolymeric material and a second portion of impermeable material, theconduit being capable of withstanding an internal pressure of at least0.5 psi at cryogenic temperatures. In certain embodiments of theinvention, the conduit may withstand an internal pressure of 10 bar orgreater.

[0032] The combination of flexibility and ability to retain pressurisedcryogenic fluid overcomes many disadvantages associated with prior artcryogenic fluid transfer tubes, which tend to be relatively inflexibleand brittle at cryogenic temperatures.

[0033] Preferably, a plurality of layers of material are superimposed oneach other to provide a multi-layered composite material possessing aspiral-shaped cross-section, formed from one or more sheets of film. Thefilm layers may be wrapped about the longitudinal axis of a mandrel. Thefilm may be circumferentially wrapped such that the film width becomesthe length of the conduit. Alternatively, long length conduits or tubesmay be constructed by helically wrapping film. Helical wrapping in twodirections may impart different properties to the tubes. In tubes formedof PTFE, the layers are bonded together by restraining the ends of thetube on the mandrel and then subjecting the assembly to temperaturesabove the crystalline melt point of PTFE. The cooled tube is thenremoved from the mandrel.

[0034] For the purposes of the present invention, the terms “porous”,and “non-porous” or “impermeable”, are defined as follows. A porousmaterial contains open cell pore spaces that allow detectable passage ofgaseous fluid across the material (e.g. as detected by a 280 ComboAnalyser supplied by David Bishop Instruments, Heathfield, East Sussex,UK). A non-porous or impermeable material does not contain continuousvoid spaces across the material thereby limiting the passage of anysubstantial amount of fluid across the material.

[0035] PTFE-based articles of embodiments of the present invention arealso preferred because of the low thermal conductivity of PTFE, which isabout 0.232 Watts/mK. Porous articles of PTFE exhibit even lower thermalconductivity. The use of low thermal conductivity materials may resultin safer articles with regard to issues such as potential for coldburns. Cryogenic fluid systems will benefit from lower thermal energyingress and resulting reduction in gas generation within the fluidtransport lines. PTFE additionally has a low heat capacity, namely 1047kJ/kgK.

[0036] The choice of precursor ePTFE film material is a function of thedesired number of layers in the final tube and tube wall thickness.

[0037] The conduit may incorporate convolutions or corrugations toenhance its bending and flex endurance characteristics. Reinforcementmembers may be incorporated helically, circumferentially, longitudinallyor by combinations thereof to enhance conduit characteristics. Thereinforcement members may be placed within or on the exterior surface ofthe tubular article. They may enhance the bending characteristics andflexural durability of the tube. Externally applied reinforcement in theform of rings or helically applied beading or filament or otherconfigurations or materials may be incorporated into the inner tubeconstruction in order to provide kink and/or compression resistance tothe article. The reinforcement materials may include, but are notlimited to, fluoropolymers (such as PTFE, ePTFE, fluorinated ethylenepropylene (FEP), etc.), metals, or other suitable materials.

[0038] The non-porous or impermeable layer or portion of the conduitwall is preferably constructed from a polymer, particularly afluoropolymer such as PTFE or FEP. These materials are reasonablydurable and flexible at cryogenic temperatures, though not as flexibleas porous ePTFE.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] These and other aspects of the present invention will now bedescribed, by way of example, with reference to the accompanyingdrawings, in which:

[0040]FIG. 1 is a part cut away perspective view of a tube in accordancewith an embodiment of the present invention;

[0041] FIGS. 2-6 are enlarged views of the section of tube wall asexposed by the cut away in FIG. 1, and illustrating various alternativetube wall constructions;

[0042]FIG. 7 is a perspective view of a step in the creation of a tubein accordance with an embodiment of an aspect of the present invention;and

[0043]FIG. 8 is a transverse sectional view of the tube form produced bythe step of FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0044] Reference is first made to FIG. 1 of the drawings, which is apart cut away perspective view of a conduit in the form of a tube 10 inaccordance with an embodiment of the present invention. The tube wall 11is formed of layers of porous and non-porous or impermeable sheetmaterial, as described below with reference to FIG. 2 to 6 of thedrawings, which are enlarged views of the section of tube wall asexposed by the cut-away in FIG. 1, and illustrate various alternativetube wall constructions.

[0045]FIG. 2 illustrates a tube wall formed with a inner base tube 12 ofexpanded PTFE (ePTFE), overwrapped with six layers of ePTFE sheet film14, followed by three wraps of ePTFE film 14 in parallel with FEP film16, followed by five wraps of ePTFE film 14, followed by another bythree wraps of ePTFE film 14 in parallel with FEP film 16, and finallyfollowed by eight wraps of ePTFE film 14.

[0046]FIG. 3 illustrates a tube wall formed with a inner base tube 12 ofexpanded PTFE (ePTFE), overwrapped fifteen wraps of ePTFE film 14 inparallel with FEP film 16, followed by a single wrap of ePTFE film 14.

[0047]FIG. 4 illustrates a tube wall formed with a inner base tube 12 ofexpanded PTFE (ePTFE), overwrapped with eleven layers of ePTFE sheetfilm 14, followed by four wraps of ePTFE film 14 in parallel with FEPfilm 16, followed by eleven wraps of ePTFE film 14.

[0048]FIG. 5 illustrates a tube wall formed with a inner base tube 12 ofexpanded PTFE (ePTFE), overwrapped with twenty one layers of ePTFE sheetfilm 14, followed by four wraps of ePTFE film 14 in parallel with FEPfilm 16, followed by a single wrap of ePTFE film 14.

[0049]FIG. 6 illustrates a tube wall formed with a inner base tube 12 ofexpanded PTFE (ePTFE), overwrapped with four wraps of ePTFE film 14 inparallel with FEP film 16, followed by twenty two wraps of ePTFE film14.

[0050] An example of a tube in accordance with an aspect of anembodiment of the present invention will now be described, following abrief description of a number or the test methods utilised to determineproperties of the materials utilised in the example.

Bubble Point and Thickness Testing for Films

[0051] Bubble point of films is measured according to the procedures ofASTM F31 6-86. The film is wetted with isopropanol (IPA).

[0052] Film thickness is measured with a snap gauge (such as Model2804-10 Snap Gauge available from Mitutoyo, Japan). Gurley AirPermeability Testing for the Film

[0053] The resistance of samples to airflow is measured by a Gurleydensimeter, such as that manufactured by W. & L. E. Gurley & Sons, inaccordance with conventional measurement procedures, such as thosedescribed in ASTM Test Method D726-58. The results are reported in termsof Gurley Number, or Gurley-Seconds, which is the time in seconds for100 cubic centimetres of air to pass through 1 square inch of a testsample at a pressure drop of 4.88 inches of water.

Isopropanol Bubble Point, Gurley Air Permeability and Tube DimensionMeasurement Testing for the Tubes

[0054] The tubes are mounted to barbed luer fittings and secured withclamps and tested intact.

[0055] The isopropanol (IPA) bubble points (IBP) are tested by firstsoaking the tubing fixtures in IPA for approximately six hours undervacuum, then removing the tubing from the IPA and connecting the tubingto an air pressure source and re-immersing the tube in IPA in atransparent container. Air pressure is then manually increased at a slowrate until the first steady stream of bubbles is detected. Thecorresponding pressure is recorded as the IBP.

[0056] The air permeability measurement is determined using a GurleyDensometer (such as a Model 4110 densometer from W. & L. E. Gurley,Troy, N.Y.) fitted with an adapter plate that allows the testing of alength of tubing. The average internal surface area is calculated fromthe measurements utilising a Ram Optical Instrument (such as a ModelOMIS II 6×12 from Ram Optical Instrumentation Inc., 15192 Triton Lane,Huntington Beach, Calif. The Gurley Densometer measures the time ittakes for 100 cc of air to pass through the wall of the tube under 4.88inches (12.40 cm) of water head of pressure.

[0057] The wall thickness and outer diameter of the tube are measuredusing the same OMIS II optical system.

EXAMPLE

[0058] An example will now be described, producing a tube wallconstruction similar to that as illustrated in FIG. 4 of the drawings.

[0059] A thin longitudinally expanded PTFE base tube 12 possessing awall thickness of 0.0051″ (0.131 mm), an inner diameter of 0.157″ (4.0mm), Gurley number of 0.9 sec, and an IBP of 0.79 psi (0.0055 MPa) isobtained. Referring to FIG. 7, this tube 12 is snugly slipped over0.250″ (6.35 mm) diameter mandrel 18.

[0060] Expanded PTFE film 14 is obtained possessing a thickness of0.0034″, (0.086 mm), a Gurley number of 37.1 seconds, and an isopropanolbubble point of 50.3 psi (0.342 MPa). All measurements are made inaccordance with the procedures previously described, unless otherwiseindicated. This ePTFE film 14 is then circumferentially wrapped over thethin ePTFE base tube 12 such that the width of the film 14 becomes thelength of the resultant tube as depicted in FIG. 8. Ten layers of film14 are wrapped around the base tube.

[0061] A sheet of continuous FEP film 16 is now placed on top of moreexpanded ePTFE film 14. This FEP 16 is 0.0005″ (0.0127 mm) in thicknessand of sufficient width and length to provide four completecircumferential wraps of the tube in parallel with the ePTFE membrane14, similar to the arrangement as shown in FIG. 4. A further elevenlayers of membrane 14 are then wrapped onto the tube to provide a totalof twenty-five layers of ePTFE membrane 14 with four layers ofcontinuous FEP 16 placed between layers eleven to fifteen of theconstruction.

[0062] The cross-sectional geometry of the layered tube construction isspiral-shaped, as indicated in FIG. 8.

[0063] The ends of the layered film and base tube construction arerestrained by restraining wires means to prevent shrinkage in thelongitudinal direction of the construction (the longitudinal axis of themandrel) during subsequent heat treatment. The restrained tubeconstruction is placed in an air oven at 375° C. for ten minutes inorder to bond the ePTFE and FEP layers and impart dimensional stabilityto the tube. The tube is allowed to cool before the wire restraints areremoved and the tube is removed over the end of the mandrel.

[0064] The finished tube length is about 25.7″ (0.653 m), outsidediameter is 0.306″ (7.772 mm) and internal diameter 0.250″ (6.35 mm).The inventive impermeable transfer tube is attached to the liquidnitrogen supply and tested in accordance with the bending diameter andcryogenic fluid permeation test as described below.

[0065] The tube example described here displayed no signs of nitrogenpermeation either before or after the bending diameter test while beingpressurised with 45 psig of nitrogen fluids.

Liquid Cryogenic Fluid Permeation Test

[0066] A liquid nitrogen fluid permeation test was developed to detectwhether liquid nitrogen permeates through a cryogen tube wall at a givenpressure.

[0067] A vacuum insulated test Dewar is obtained from A S Scientific Ltd(Abington, Oxford, UK). The Dewar has a holding capacity of ten litresof liquid nitrogen and is fitted with a burst disc (Elfab Hughes) asover pressure protection. Discharge and vent valves are ½″ bore ballvalves supplied by A S Scientific. Immediately after the test dischargevalve a ½″ BSP to ¼″ Swagelok compression fitting (supplied by South ofScotland Valve and Fitting Company, Irvine, Scotland) was fitted. Eachend of the test sample had a piece of stainless steel tube inserted(0.95″ long×0.25″ od×0.215″ id) to half its length and fastened there bymeans of an Oetiker Crimp fastening by Oetiker, Inc, Livingston, N.J.,U.S.A. The remaining exposed insert length allowing for the attachmentof the Swagelok compression fitting. The test tube has another stainlesssteel tube inserted into the other end to which was attached, by meansof another Oetiker Crimp and Swagelok compression assembly, a pistoncontrol valve (Swagelok, part number SS-1GS4). From the exit of thisvalve was fitted 6 m of polyethylene tube (0.16″ bore, 0.248″ outsidediameter). This tube was used to lead the exhaust gas from the testassembly away from the vicinity of the gas analyser (to another room).

[0068] Liquid nitrogen is added to the lumen of tested tubes andpressurised to a predetermined pressure, selected on the basis of theintended application of the tubes. The tube wall is probed with a{fraction (1/16)}″ (1.6 mm) bore silicone tube connected to a gasanalyser (model 280 combo, David Bishop Instruments, Heathfield, EastSussex, England). The tube was used to probe along the length of thetube wall to measure the oxygen content of the air at the tube wall.Typically four or five measurements would be taken over a period ofabout one minute. If there is a drop in oxygen content of the airsampled then nitrogen has permeated through the tube wall.

[0069] Following a bending diameter test (described below) a furtherexamination of the tube wall is carried out to determine if flexure ofthe tube wall has resulted in damage to the wall internal structure thusallowing permeation to start.

[0070] Whereas this test was developed specifically for testing tubes,the same principles may be applied to create a test for the examinationof the properties of other shapes of materials. The important elementsof the test include: controlled flexure or bending of the tube andability to measure the pressure required to force a mass of liquidnitrogen to permeate the tube wall.

Bending Diameter Test

[0071] Five minutes after the opening of the Dewar valve, whichinitiates the cryogenic fluid permeation test, the transfer tube iswrapped around the outside of a hollow non-metallic, typically polymeric(for example, nylon) cylinder to determine the diameter at which thetube wall will rupture or allow permeation of fluids. Liquid nitrogencontinues to flow through the tubes during the test. The tube isexamined for evidence of kinking. “Kinking” is defined as a crease inone or more of the tubular components. Following a bending test the tubeis again tested to assess for initiation of permeation of cryogen. Thetube is also visually examined for evidence of fracture, to determine ifthe wrapping had compromised the ability of the tube to hold liquid.

[0072] It will of course be apparent to those of skill in the art thatthe above described embodiments and example are merely exemplary of thepresent invention and that various modifications and improvements may bemade thereto without departing from the scope of the present invention.

1. A method of transferring a cryogenic fluid, the method comprisingpassing a cryogenic fluid through a flexible conduit having a wallformed of a first layer of a porous polymeric material and a secondlayer formed of an impermeable material.
 2. The method of claim 1,wherein the first layer is selected to have a heat capacity of less than2.251×10⁶ kJ/m³K.
 3. The method of claim 2, wherein the first layer isselected to have a heat capacity of less than 1.75×10⁶ kJ/m³K.
 4. Themethod of claim 3, wherein the first layer is selected to have a heatcapacity of less than 1.25×10⁶ kJ/m³K.
 5. The method of claim 4, whereinthe first layer is selected to have a heat capacity of less than0.75×10⁶ kJ/m³K.
 6. The method of claim 5, wherein the first layer isselected to have a heat capacity of less than 0.5×10⁶ kJ/m³K.
 7. Themethod of any of the preceding claims, wherein the first layer isselected to have a thermal conductivity of less than about 0.232Watts/mK.
 8. The method of claim 7, wherein the first layer is selectedto have a thermal conductivity of less than about 0.186 Watts/mK.
 9. Themethod of claim 8, wherein the first layer is selected to have a thermalconductivity of less than about 0.139 Watts/mK.
 10. The method of claim9, wherein the first layer is selected to have a thermal conductivity ofless than about 0.113 Watts/mK.
 11. The method of claim 10, wherein thefirst layer is selected to have a thermal conductivity of less thanabout 0.066 Watts/mK.
 12. The method of any of the preceding claims,wherein the conduit is selected to have a heat capacity of less than2.251×10⁶ kJ/m³K.
 13. The method of claim 12, wherein the conduit isselected to have a heat capacity of less than 1.75×10⁶ kJ/m³K.
 14. Themethod of claim 13, wherein the conduit is selected to have a heatcapacity of less than 1.25×10⁶ kJ/m³K.
 15. The method of claim 14,wherein the conduit is selected to have a heat capacity of less than0.75×10⁶ kJ/m³K.
 16. The method of claim 15, wherein the conduit isselected to have a heat capacity of less than 0.5×10⁶ kJ/m³K.
 17. Themethod of any of the preceding claims, wherein the conduit is selectedto have a thermal conductivity of less than about 0.232 Watts/mK. 18.The method of claim 17, wherein the conduit is selected to have athermal conductivity of less than about 0.186 Watts/mK.
 19. The methodof claim 18, wherein the conduit is selected to have a thermalconductivity of less than about 0.139 Watts/mK.
 20. The method of claim19, wherein the conduit is selected to have a thermal conductivity ofless than about 0.113 Watts/mK.
 21. The method of claim 20, wherein theconduit is selected to have a thermal conductivity of less than about0.066 Watts/mK.
 22. A method of transferring a cryogenic fluid betweentwo relatively movable locations, the method comprising passing acryogenic fluid through a flexible conduit having a wall formed of afirst layer of a porous polymeric material and a second layer formed ofan impermeable material.
 23. A method of storing a cryogenic fluid, themethod comprising placing a cryogenic fluid in a container having a wallformed of a first layer of a porous polymeric material and a secondlayer formed of an impermeable material.
 24. A method of insulating acryogenic fluid container having a wall formed of a first layer of animpermeable material, the method comprising providing the wall with asecond layer of a porous polymeric material.
 25. The method of claim 24,wherein the second layer is provided internally of the first layer. 26.The method of claim 24, wherein the second layer is provided externallyof the first layer.
 27. A flexible cryogenic fluid transfer conduitcomprising a wall formed of a first portion of a porous polymericmaterial and a second portion comprising a plurality of layers of coiledimpermeable sheet.
 28. A flexible cryogenic fluid transfer conduitcomprising a wall formed of a inner first portion comprising a pluralityof layers of porous polymeric sheet and an outer second portioncomprising a plurality of layers of impermeable sheet, the impermeablesheet being of smaller thickness than the porous polymeric sheet.
 29. Aflexible cryogenic fluid transfer conduit comprising a wall formed of afirst portion of porous polymeric material and a second portion ofimpermeable material, the conduit having a diameter of less than 25.4mm.
 30. A flexible cryogenic fluid transfer conduit comprising a wallformed of a first portion of porous polymeric material and a secondportion of impermeable sheet material having an axially extending edge.31. A flexible cryogenic fluid transfer conduit comprising a wall formedof a first portion of a seamless porous polymeric tube and a secondportion of impermeable material.
 32. A flexible cryogenic fluid transferconduit comprising a wall formed of a first portion of porous polymericmaterial and a second portion of impermeable material, at cryogenictemperatures the conduit having a flexibility, as determined by the benddiameter test set out above, of 20 to 1 or less.
 33. The conduit ofclaim 32, wherein, at cryogenic temperatures, the conduit has aflexibility of 10 to 1 or less, that is the bend diameter of the conduitmay be less than 10 times the diameter of the conduit.
 34. The conduitof claim 33, wherein, at cryogenic temperatures, the conduit has aflexibility of 5 to 1 or less, that is the bend diameter of the conduitmay be less than 5 times the diameter of the conduit.
 35. A flexiblecryogenic fluid transfer conduit comprising a wall formed of a firstportion of porous polymeric material and a second portion of impermeablematerial, the conduit having a heat capacity of less than 2.251×10⁶kJ/m³K.
 36. A flexible cryogenic fluid transfer conduit comprising awall formed of a first portion of porous polymeric material and a secondportion of impermeable material, the conduit having a thermalconductivity of less than about 0.232 Watts/mK.
 37. A flexible cryogenicfluid transfer conduit comprising a wall formed of a first portion ofporous polymeric material and a second portion of impermeable material,the conduit being capable of withstanding an internal pressure of atleast 0.5 psi.
 38. A flexible cryogenic fluid transfer conduitcomprising a wall formed of a first portion of a porous polymericmaterial and a second portion comprising a smooth-walled layer ofimpermeable material.
 39. The conduit of any of claims 27 to 38, whereinthe porous polymeric material comprises expanded PTFE.
 40. The conduitof any of claims 27 to 39, wherein the impermeable material comprises afluoropolyer.
 41. The conduit of claim 40, wherein the impermeablematerial comprises PTFE.
 42. The conduit of claim 40, wherein theimpermeable material comprises a copolymer of hexafluoropropylene andtetrafluoroethylene.
 43. The conduit of claim 40, wherein theimpermeable material comprises a copolymer of tetrafluoroethylene andperfluoropropyl vinyl ether (PFA).